Float32Array#subarray instead of [].proto.slice

Also use a loop instead of Math.max

slice vs subarray: http://jsperf.com/float32array-array-slice-call-vs-subarray
for loop vs Math.max: http://jsperf.com/find-max-in-float32array
getPeaks (new vs old): http://jsperf.com/wavesurfer-getpeaks